FIG. 1 shows a prior art device enabling a detection of whether a magnetic read/write head is in a short-circuit state or open circuit state at one of its input terminals. The assembly presented is a so-called H assembly. In its central part, it has the components Rh, Lh of a magnetic read/write head, representing its internal series resistor and its self-inductive impedance. Depending on the direction of a bit to be written in a magnetic medium placed before the head (near the impedance Lh), a pulse signal WR+ or WR- is applied to the bases of transistors respectively Q1 and Q6 or Q2 and Q5. These NPN type transistors are cascade-connected with each other and series-connected in an intermediate position with the head. If one of these pulses is positive it turns the corresponding pair of transistors on. The other signal is then null and places the other transistors in an open circuit. The result thereof is that the current flows into the self-inductance Lh in a direction dictated by the polarity of the signals applied.
In the example, this current is produced by a voltage source Vcc and is limited by a current source Iwr mounted at the foot of the H circuit before the ground. For practical reasons, it may become necessary to provide for the addition, in the transmission of the signals, of a negative shift in the control voltage of the transistors of the pair of transistors to be made conductive. These considerations which are only related to the technology of the transistors Q1, Q2, Q5 Q6 in no way modify their principle.
To ascertain that one of the terminals TFP or TFN of the head is either shorted-circuit or in an open circuit, each of these terminals is furthermore connected to the supply voltage Vcc by a transistor, Q3 or Q4. This transistor Q3 or Q4, which is of same type as the preceding transistors, is itself respectively series-connected with a biasing resistor R1 or R2. The transistors Q3 and Q4 are controlled by the control signals of the transistors Q1 and Q2 respectively. Measurement voltages are then taken at midpoints between the resistor R1 and the transistor Q3 or between the resistor R2 and the transistor Q4 respectively.
When the head is accurately connected, at the time of the application of one of signals WR+ or WR-, the voltage of one of these midpoints drops (without however reaching the ground potential) while the other one remains at the voltage Vcc. On the other hand if there is a short-circuit, on the side where this short-circuit is located the voltage falls to zero and this null signal is detected as a signal that reveals the defect. On the contrary, if one of the two terminals is in an open circuit condition, then the voltage at both midpoints remains equal to Vcc at the time of the application of the pulses WR+ or WR-.
This mode of operation has the main disadvantage of being dependent on the presence of the write signals WR+ or WR-. It thus enables the detection of the correct operation of the head only when one mode of writing is already in progress. In addition, all the parasitics appearing on the supply voltage and/or on the signals WR+ and WR- give rise to false detections. This means that the reliability of this detection device is relatively poor.